The present invention relates generally to the field of video compression. More specifically, it relates to a system for using knowledge of the adjustment of a video camera to optimize the use of resources for compressing a video signal generated by the video camera.
Video surveillance cameras are commonly used to monitor premises for security purposes. Typically one or more video cameras are placed at various locations to be monitored. The output of the cameras may be viewed or recorded at a central station. It is also possible to have a number of video cameras distributed at locations which are remote from the central station. For example, cameras may be placed in several stores at various locations in a city and monitored from a central station. Remote cameras could also be placed at outlying parts of an airport and monitored centrally.
In such systems, it is necessary to transmit the information acquired by the video cameras to the central monitoring location. For this reason, it is often desirable to compress the video data so that it can be transmitted over a communications channel having a relatively narrow bandwidth.
Known video compression systems involve two basic forms of compression processing spatial and temporal. Spatial processing compresses information by transforming the picture elements within a particular frame of a video signal in accordance with a compression algorithm, thereby reducing the amount of information required for reproduction of the frame. In contrast, temporal processing takes into account the way in which information is changing with time. It therefore reduces the amount of information required for reproduction of a frame by taking into account changes in the picture which occur from frame to frame. These changes are reflected in motion vectors which are generated and transmitted instead of the actual contents of the video frames. A description of an implementation of spatial and temporal processing can be found in the MPEG compression recommendation ISO/IEC 1172-2 (referred to herein as the MPEG Standard).
The MPEG Standard is one of several well known standards for video processing. Conventional MPEG encoders allow the degree of spatial processing to be varied, for example to conserve memory by adjusting the quantization of information in a particular frame. Such encoders also have the facility to detect motion of the picture from frame to frame and adjust the degree of temporal processing (i.e. adjust the motion vectors).
Within a scene being monitored by a video camera, motion can occur due to movement of the subject (e.g. a person traversing the field of view of the camera), or as a result of movement of the camera (i.e. due to the panning, tilting, zooming or focusing of the camera). When the picture moves, the movement information must be extracted in order to generate motion vectors. Systems of the prior art (e.g. systems using MPEG-type compression) which perform temporal processing to convey motion information require relatively large amounts of memory space and computational power.
The present invention is directed to the use of known information about the movement of the video picture caused by movement due to the camera to reduce the computational and memory overhead required for compression of video data. Specifically, it uses information generated as a result of adjustments to the camera to adjust and thus trade off spatial processing against temporal processing. Rather than extracting the information from the video pictures, it obtains the information from the means by which the camera is actually being controlled.